He has psoriatic arthritis, which attacks the joints or, in his words "turns the body against itself.
"Mine is mobile, starts in one joint one day, then another joint.
"It affects my whole body."
Shearsby said they were only a small team - the Americans had around 100 - but they will give it their all and compete in as many events as possible.
"I'm an expert in nothing," he said.
He will taking on the 50m swimming freestyle, but, unsurprisingly in the military, he's discovered he's been "volunteered" for the 50m freestyle relay.
He will also tackle the archery, shotput, indoor rowing, volleyball, wheelchair rugby and wheelchair basketball.
Shearsby, 42, started his military career in the army but carrying a pack became problematic when his illness was diagnosed. He now works for the navy, in Joint Forces Headquarters at Upper Hutt.
He likes Featherston for the climate and the house prices, he said. Plus it feels right - he grew up on a farm.